In 1923 Freud introduced into psychoanalytic theory a threefold division of the mind that produced a bounty of fresh developments as it revived old lines of investigation and sowed seeds for new ones. Previously, Freud had used, in constructing his theory, a topographical model that was based on the seemingly simple opposition between the mind's conscious and unconscious parts. The conscious part he located at and near the surface of the mind, where via sensory stimulation it came into contact with its environment and where deliberation and decision making about how best to adapt to the environment occurred. The unconscious he located in the mind's interior, where inherited drives, which is to say, instincts, had their source and where many thoughts and ideas the awareness of which would produce distress were buried. By 1923, however, this model had proved unsatisfactory, and Freud abandoned it for a structural, one might say bureaucratic, model according to which the mind was divided into three agencies - ego, id, and superego - and the interactions among them were the basis for explanations of the psychological phenomena Freud sought to understand. This change in explanatory model, contrary to what one might expect, did not result from a discovery of new phenomena, nor did it represent a movement to a deeper level of theory through introduction of novel theoretical entities.